Conductance meter



April 3, 1951 w. A. Mocool.

CONDUCTANCE METER Filed Oct. l2, 1948 xvi( Patented Apr. 3, 1951 CONDUCTANCE METER William A. McCool, Hyattsville, Md., assigner to Boonton Radio Corporation, Boonton, N. J., a corporation of New Jersey Application October 12, 1948, Serial No. 54,118

This invention relates to a radio frequency impedance measuring device, and moretparticularly toa device for measuring both the conductive and susceptive components of a specimen by substitution methods. The specimens may be resistors, inductors, choke coils, transmission lines, radio frequency transformers, capacitors or insulating materials arranged between metal electrodes, or entire radio frequency circuits.

For convenience of description, the device will 'be termed a conductance meter because of the facility with which it may be employed for the measurement of conductance.

The prior measuring techniques for evaluating the radio frequency loss characteristics of high quality insulating material did not alford high accuracy with rapid and simple methods, and with relatively inexpensive equipment. For example, the round-robin testing of a group of typical ceramic insulating materials by thirteen testing agencies showed differences of as much ,as ,1,1% in the dry power factor measurements of different agencies even though the specimen `material was not of the exceptionally low-loss variety. ments by the known methods and equipment has In general, the accuracy of measuredecreased with' decreases in power factor or conductance, and with increases in the radio frecuencies at which the measurements were made.

Objects of the present invention are to provide radio irequency impedance measuring devices which afford high accuracy of measurement of low conductance values at high radio frequencies and, if desired, at different radio frequencies; which are characterized by simple and rapid operational methods; and which are of relatively simple and inexpensive construction. An object is to provide impedance measuring devices which are stable in operation and retain their initial calibration. over long periods of use. An object is to'provide impedance measuring devices oper- "ating on the resonant rise principle which aioid direct or substantially direct readings, by substitution methods, of the conductive and susceptive components of test specimens. A further object is to provide impedance measuring devices '7 Claims. (Cl. F75-183) in which the calibrated and adjustable standard of conductance takes the form of a diode in series with an adjustable direct current resistance. I.

These and other objects and the advantages of the invention Will be apparent from the following specication when taken with the accompanying drawing in which:

Fig. 1 is a simplified and schematic diagram of the basic circuit of a conductance meter embodying. the invention;

` ance.

Fig. 2 is a circuit diagram of a multirange conductance meter; and

Fig. 3 is a curve sheet showing the several conductance measuring ranges for a particular embodiment of the invention.

In Fig. 1, the basic measuring circuit is shown as comprising a tunable circuit, provided by an inductor L and an adjustable capacitor C, across which a diode D and an adjustable resistor R are connected in series, the resistor being shunted by a radio frequency by-pass condenser C. As shown schematically, the resistor R is preferably a potentiometer connected between ground and the cathode of diede D, the sliding tap of the potentiometer being grounded. The potentiometer shait carries a graduaed dial or, as indicated schematically, a pointer cooperating with a scale Se graduated in conductance values, the zero scale graduation corresponding to maximum oonductance of the series diode circuit, i. e. to adjusinent' of the potentiometer tap to by-pass a preselected portion oi the potentiometer resist- The graduations of scale Sd thus represent changes in the conductance of the diode circuit as the ohmic resistance ofthe resistor R is increased to its maximum value from its preselected minimum value established by the allowable loading of the measuring circuit. The tuning condenser C is provided with a scale Se graduated values of capacitance. The measuring circuit is energized by an oscillator 0v which, as shown schemaically, may be tunable over a frequency range, through any desired type of coupling such as, for example, a coil L coupled to the coil L of the measuring circuit. The particular type oi the oscillator Q is not important but it should be of a known type which is self-regulated to deliver a highly stabilized output voltage or current. Terminals T, T are connected to the opposite ends of the tunable circuit L, C, and a test specimen having conductive component GX and a susceptive component BX may be connected across these test terminals. The total circuit losses, exclusive of the diode circuit, are lumped and indicated schematically as a conductance G0 in shunt with the tuned circuit L, C, but it is not necessary to determine the value G0 since the value GX oi a test specimen is determined by a substitution method which is independent oi the total measuring circuit parameters. A vacuum tube voltmeter V is shunted across the tunable circuit L, C to afford an indication of the resonance or ofi-resonance condition of the circuit as condenser C is adjusted.

rlhe circuit is conditioned for ar measurement, before the test specimen connected across terminals T, T, by adjusting the conductance of the diode circuit to its maximum value, i. e. by adjusting the potentiometer resistor R to its selected minimum value. This corresponds to a Zero conductance reading on the graduated scale Se. The condenser C is then adjusted for resonance, i. e. to obtain a maximum reading at voltmeter V. This voltage reading is noted, and the test specimen is then connected across terminals T, T.

The voltage across the circuit L, C is thereby altered since the susceptive component Bx of the test specimen shifts the resonant frequency of the tuned circuit L, C to a new value, and the conductance GX alters the load across the tuned circuit. Condenser C is adjusted to obtain a new maximum voltage indication at voltmeter V, and the change in capacitance a5 read from the scale Se determines the susceptance Bx of the test specimen. The potentiometer resistance r R. is then adjusted to'obtain the previouslynoted voltage reading at voltmeter V, thereby reducing -the elective conductance Gd by an amount equal to the conductance Gx of the test specimen.

-rectly in values of the changes in the total radio 'frequency conductance of the diode circuit as Ia. function of the changes in the direct current resistance of potentiometer R.

The voltage change developed across the meas- 'ing circuit by specimens of low conductance is too small to permit an accurate resetting of even a vacuum tube type of voltmeter V to a previ- :ously obtained reading and, for high sensitivity, a :balanced vacuum tube voltmeter circuit as :shown in Fig. 2 is employed. The Fig. 2 cir- 'cuit also differs from the basic circuit of Fig. l in that provision is made for measuring conductance values in a plurality of ranges. Such elements of the Fig. 2 circuit as are, or may be, 4identical with elements of'the Fig. 1 circuit are identified by like reference characters but will not be described in detail.

For multirange measurements, the single adjustable resistor R of the Fig. l circuit is here replaced by an adjustable potentiometer resistance R and a bank of xed resistors R1, R2, R3 of different values, any one of which may be connected in series with adjustable resistor R' by means of a range change switch S.

The voltage balance System for obtaining an accurate indication of the resonance condition Aof the measuring circuit LC includes a voltage rectier circuit in broken line rectangle RV for "deriving an adjustable direct current voltage from the oscillator O, a similar voltage rectifier '-.circuit in broken line rectangle MV for rectify- .ing the alternating current Voltage across the nreasuring circuit, and an amplitude balance in- 'tical construction with diodes I, 2, respectively,

which are connectedto ground :through cathode resistors 3, :i which are'by-passed for radio frequency lby condensers 5, 6, respectively. The

`abalance indicator B includes a pair of amplifier 4 tubes 1, 8 with cathode resistors 9, III, respectively, and a microammeter II connected across the cathodes in series with an adjustable sensitivity-control resistor I2.

The grid of tube 'l is connected by lead I3 to an adjustable tap IIS on cathode resistor 9 of diode i, and the grid of tube Il is connected by lead I5 to the cathode of the diode 2. The microammeter II is of the zero center type and the voltages developed across cathode resistors 9 and I0 tend to deflect the instrument pointer in opposite directions.

rThe Fig. 2 circuit is conditioned for a conductance measurement by adjusting resistance I2 to its maximum value (minimum sensitivity of instrument II) for protective purposes, adjusting resistance R' to its minimum value, and then adjusting condenser C to obtain a maximum deflection of the instrument pointer in the sense corresponding to increased voltages on the measuring voltage diode 2. The tap I4 is then adjusted along the reference voltage resistor 3 to set the pointer of instrument I I to its vzero position. During this balancing step, the sensitivity control resistor I2 is progressively reduced in eifective value to increase the instrument sensitivity, thereby affording a high resolution of the nal conductance evaluation. These adjustments afford a highly accurate balance of the voltage across the measuring circuit, at resonance, against an equal reference voltage derived from the oscillator O.

The resistor I2 is again set for low instrument sensitivity, the test specimen is connected across theterminals T, T, and condenser C is adjusted 'to resonate the measuring circuit again, i. e. is

adjusted to effect a maximum displacement of the pointer of instrument II 'from'its zero position as the sensitivity of the'instrument is progressively increased by adjustment lof'resistor I2. `This change in `the capacitance is noted as the value of the susceptance Bx of the test specimen.

rIhe resistance R in the cathode circuit of diode D is then adjusted to reset the instrument lpointer to its initial zero position, and the change in vdiode circuit conductance due to the change in value of resistance R', as indicated on conductance scale Se, is equal to the conductance of the test specimen.

The particular measuring ranges which are obtained with an embodiment of the invention as illustrated-in Fig. 2 depend upon the range `of adjustment of the resistance or conductance values of the several series cathode circuits ofthe diode D. The number of and the relative values of the fixed resistors of the cathode circuit of diode D may beso selected as to afford conductance measurements in a plurality of overlapping ranges. For simplicity in illustration only three xed resistors which afford conductance measurements in spacedranges are illustrated.

In a practical constructional embodiment of the invention, in which diode D Wasa tube of the Y6H6 type, the values of the cathode resistorsof the tube D were:

Ri=1.22 megohms, range I R2=335 kilohms, range II R3=l00 kilohms, range III Aof direct current load resistance of the diode'D and corresponding changes in. diode. circuitl con- 5 ductance due to variation, in each measuring range of resistor R from zero to 100,000 ohms.

As above described, the sequence of steps in determining the conductance of a test specimen or circuit is to resonate the measuring circuit and balance the meter Il at zero with the test specimen disconnected, the range switch S set for the desired range, and the potentiometer R set for minimum resistance (maximum loading or maximum conductance). rlhe test specimen or circuit is then connected across terminals T, T, the measuring circuit is resonated, and a conductance equal to the conductance Gx of the test specimen or circuit is removed from the measuring circuit by increasing the diode load resistance, by means of potentiometer R', to re- .balance the meter ,Il at zero. An alternative method of calibration and use of the apparatus is to resonate the measuring circuit and balance the meter I l at zero with the test specimen connected across the terminals T, T, and with the diode conductance at a minimum value (maximum diode load resistance). The test specimen will then be removed and conductance added,

by increasing the effective value of potentiometer R until balance is again attained.

It is to be understood that the invention is not limited to the particular circuit herein illustrated and described, or to specified values of circuit elements employed in one practical embodiment of the invention, and that various changes in circuit arrangement and relative values of circuit elements fall within the spirit and scope of the invention as set forth in the following claims.

I claim:

l. In a device for measuring radio frequency impedance, a tunable radio frequency circuit comprising an inductor and a tuning capacitor in parallel, a source of radio frequency energy coupled to said radio frequency circuit, an adjustable conductance connected across said radio frequency circuit and comprising a diode in series with resistance means of adjustable magnitude, a radio frequency capacitance bypassing said resistance means, indicating means responsive to the radio frequency voltage across said radio frequency circuit, a pair of measuring terminals connected to the opposite ends of said radio frequency circuit, and a scale adjacent said resistance means calibrated in values of change in the total radio frequency conductance of said diode and resistance means with changes in the magnitude of said resistance means.

2. In a device for measuring radio frequency impedance, the invention as recited in claim 1, wherein said indicating means includes means for rectifying the radio frequency voltage across the radio frequency circuit, and a direct current instrument responsive to variations in the magnitude of the rectified voltage.

3. In a device for measuring radio frequency. impedance, the invention as recited in claim l, wherein said indicating means includes an electronic comparator circuit, and means for impressing upon said comparator circuit in opposition a direct current voltage varying in magnitude with the radio frequency voltage across said radio frequency circuit and a direct current voltage of adjustable magnitude derived from said source of radio frequency energy.

4. In a device for measuring radio frequency impedance, the invention as recited in claim l, wherein said indicating means includes an electronic comparator circuit comprising a pair of vacuum tubes each having a control grid and plate cooperating with a cathode, load impedances rconnected between the plate and cathode of the respective tubes, a direct current measuring instrument connected between said load impedances, means including a rectifier connected across said radio frequency circuit for impressing upon the grid of one tube a direct current voltage varying with the radio frequency voltage across said radio frequency circuit, and means for impressing upcn the grid of the other tube a direct current voltage of adjustable magnitude.

5. In a device for measuring radio frequency impedance, the invention as recited in claim 4, in combination with means for adjusting the sensitivity of said direct current measuring instrument. Y

6. In a device for measuring radio frequency impedance, the invention as recited in claim 4, wherein said last means includes a rectifier circuit connected across said source of radio frequency energy, a resistor in said rectier circuit, and a tap adjustable along said resistor and connected to the grid of said other tube.

7. In a device for measuring radio frequency impedance, the invention as recited in claim 1, wherein said resistance means includes a resistor of adjustable magnitude, a plurality of fixed resistors of different magnitudes, and switch means for connecting said resistor of adjustable magnitude in series with a desired one of said fixed resistors.

WILLIAM A. MCCOOL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS i Number Name Date 1,789,369 Meissner Jan. 20, 1931 2,043,241 Eyer June 9, 1936 2,071,607 Bjorndal Feb. 23, 1937 

